This invention relates to an electronic engine control system for setting a max. possible power on the basis of the flight condition data stored in a data cube for various engine ratings and a variety of power-limiting thermodynamic and mechanical engine parameters.
The maximum power provided by an aircraft engine without transgression of its thermal and mechanical loadability depends essentially on the respective flight condition which is adequately defined firstly by the flight altitude, secondly by the ambient temperature and thirdly by the flight Mach number. An important, power-limiting thermodynamic engine parameter for the power control of aircraft engines is the turbine gas temperature which is known to be applied such that—on the basis of a constant flight Mach number and a constant ambient pressure—a maximum thrust is determined which corresponds to the respective ambient air temperature, but is limited by the critical turbine gas temperature. Since, however, engine power is sufficiently high in a low ambient temperature range and since the maximum thrust limited by the turbine gas temperature is only achieved at a certain ambient temperature level and decreases continually from this kink point, the respective data will be used for power control. This means that the max. permissible turbine gas temperature, as a critical thermodynamic engine parameter, the maximum thrust, which remains constant in a low ambient temperature range up to a certain ambient temperature level (flat-rated thrust), and the kink point are determined individually for various engine ratings—take-off, climb, cruise or engine failure. This situation is shown in FIG. 3 of the illustration.
For power control via fuel supply, one function generator is provided for each engine rating, starting out from a certain setting of the power lever, actually in dependence of the flight condition defined by flight altitude, flight Mach number and ambient temperature, as described in Specification U.S. Pat. No. 5,315,819, for example. The function generators are fed with input signals for flight altitude, Mach number and ambient temperature. Corresponding output signals are supplied in response to the respective input signal, with the output signals changing with the flight condition. Each function generator comprises a three-dimensional table (data cube) whose functional range is defined by the flight Mach number (MN), the ambient temperature in the form of the difference from the ISA temperature (DTAMB), and the flight altitude (ALT) in the form of a flight altitude-related pressure. See FIG. 1. The three-dimensional table shows a target power based on the input signals for Mach number, flight altitude and ambient temperature, with each function generator performing a triple interpolation if the input signals fall between the values unambiguously defined by the table (supporting points) in order to derive the corresponding target power (target torque). For example, data are filed in the data cube related to the respective engine rating such that tables are created for different flight altitudes which map the Mach number as a function of the ambient temperature. For programming reasons, all altitude tables must, however, have exactly the same supporting points (discrete values) between which interpolation for the determination of intermediate power values must be made if the input signals for ambient temperature, Mach number and flight altitude depart from the supporting points. These specified interpolation rules are robust, reliable calculation routines which ensure a certain degree of safety.
With the above-described control system, the thrust curve is unambiguously defined by a minimum of three supporting points, actually the coldest day, the kink point and the hottest day, provided that the maximum thrust is determined on the basis of only one power limiter, here the turbine gas temperature, in relation to the ambient temperature at constant Mach number and flight altitude. Assuming a constant distribution of the supporting points over Mach number and flight altitude, the number of supporting points can accordingly be kept small. With exactly one kink point for all flight altitudes being defined for each engine rating and one power limiter, the number of data points stored in a data cube lies within narrow, controllable limits as regards storage effort and storage capacity. If, however, under the aspect of maximum engine performance at any flight condition, several power-limiting parameters are to be applied and, consequently, the kink point is not defined firmly, the number of supporting points stored in the data cube must, on the basis of permissible calculation routines in connection with the required equality of the supporting points, be many times higher to exactly define the kink point to prevent the engine from exceeding its thermodynamic and mechanical load limits and to enable the engine to deliver the desired high power. The high quantity of data entails, however, many times the capacity of very costly data storage. Also, the calculation effort is increased significantly and is so time-consuming that the safety of the flight is no longer ensured. The processing effort for the creation of the data cubes required for each engine rating is high, this effort also being necessary each time a limiting parameter is changed.